La luz trasera funciona bien.

Because luz is a feminine noun in Spanish, so it takes the feminine article la.

• la luz = the light
• una luz = a light

Even though the word ends in -z instead of the more typical -a, it is still feminine. Grammatical gender in Spanish does not always match the ending perfectly, so this is something you learn with the noun.

In Spanish, descriptive adjectives usually come after the noun.

• luz trasera = rear light
• literally: light rear

This is normal Spanish word order. English often puts the describing word first, but Spanish usually puts it after the noun.

Because the adjective has to agree with the noun it describes.

Here, the noun is luz, which is feminine singular, so the adjective must also be feminine singular:

• luz trasera = feminine singular
• foco trasero = masculine singular
• luces traseras = feminine plural

So trasera matches luz.

Trasera means rear, back, or located in the back.

In this sentence, luz trasera refers to a light at the back of something, often a vehicle. Depending on context, English might translate it as:

• rear light
• taillight
• back light

So trasera is describing the position of the light.

Yes. In context, especially with vehicles, luz trasera can refer to a rear light or taillight.

That said, native speakers may also use more specific terms depending on the country and situation, such as:

• luz trasera
• luz de atrás
• calavera(s) in some regions of Mexico for taillights

So luz trasera is a clear, standard phrase, but regional vocabulary may vary.

Funciona comes from funcionar, which means to function, to work, or to operate.

So:

• La luz trasera funciona bien = The rear light works well.

Spanish often uses funcionar for machines, devices, systems, and parts of vehicles.

Examples:

• El motor funciona bien.
• Mi celular no funciona.
• Los frenos funcionan perfectamente.

Spanish usually does not need subject pronouns when the subject is already clear.

Here, la luz trasera is the subject, so adding ella would usually sound unnecessary.

• La luz trasera funciona bien. = normal
• Ella funciona bien. = possible only if the context already makes it very clear what ella refers to

Spanish often leaves out pronouns because the verb form already helps identify the subject.

It is the present indicative, third person singular.

The verb funcionar is conjugated like this in the present:

• yo funciono
• tú funcionas
• él/ella/usted funciona
• nosotros/nosotras funcionamos
• ellos/ellas/ustedes funcionan

In the sentence, la luz trasera is a singular subject, so the verb is funciona.

Bien means well.

It is an adverb, so it modifies the verb funciona:

• funciona bien = works well

Compare:

• bien = well
• bueno / buena = good

This difference is important:

• La luz funciona bien. = The light works well.
• La luz es buena. = The light is good.

Because bien describes how it functions, not what the light is like.

• bien = adverb, modifies a verb
• buena = adjective, modifies a noun

In your sentence:

• funciona bien = functions well

If you said la luz trasera es buena, that would mean something more like the rear light is good, which is a different idea.

Yes, but it means something slightly different.

• La luz trasera funciona bien = The rear light works well.
• La luz trasera está bien = The rear light is fine / okay.

Funciona bien focuses on operation or performance.
Está bien is broader and could mean it is okay, in good condition, or not damaged.

So they are related, but not identical.

Yes, but the difference is small.

• luz trasera sounds a bit more technical or standard
• luz de atrás sounds a bit more everyday and literal: the light in the back

Both are understandable. In many contexts, either one works fine.

With the article included, it is:

La luz trasera funciona bien.

A simple pronunciation guide: lah looz trah-SEH-rah foon-see-OH-nah bee-EHN

A few useful notes for Latin American pronunciation:

• z in luz is usually pronounced like s
• c before i in funciona sounds like s
• r in trasera is a tapped Spanish r

So luz sounds like loos, not like the English word luz might look to an English speaker.

Normally, no. You usually need the article here:

• La luz trasera funciona bien.

Without la, it sounds incomplete in most normal contexts. Spanish often uses definite articles where English may or may not use the, and in this sentence the article is expected.

Grammatically, it refers to one specific rear light, because everything is singular:

• la luz
• trasera
• funciona

If you wanted the plural, you would say:

• Las luces traseras funcionan bien. = The rear lights work well.

So the original sentence is singular.